1. Field of the Invention
This invention relates to roof bolts and, more particularly, to a roof bolt which is positioned in a bore hole drilled in a rock formation in a mine roof and which is held in place within the bore by both a mechanical anchor and a quick-setting resin.
2. Description of the Prior Art
It is a well-established practice in underground mining work, such as coal mining, tunnel excavation or the like, to reinforce or support the mine roof to prevent rock falls or cave-ins. A common means presently used to support mine roofs is an elongated bar which is inserted into the rock formation above the mine roof in a bore hole and which is securely fixed to the bore hole by an anchoring means such as a mechanical anchor, a quick-setting resin which surrounds the end of the bolt within the hole, or both. The roof bolt, placed under the tension, is used to hold a metal support plate in close engagement with the roof.
The roof bolt described in U.S. Pat. No. 4,655,645 combines the features of a mechanical anchor and resin bonding but also provides positive and complete mixing of the resin components by an additional mixing mechanism. This arrangement is relatively inexpensive and easy to manufacture, forces the resin upwards along the bolt during mixing, more violently mixes the resin for a shorter mix time, and eliminates the use of a two-position coupling or delay mechanism. While this arrangement functions well in hard rock-like formations, it has been found to be less effective in soft strata. Further, at times even this type of mine roof bolt does not properly set in the rock formation because of slippage of the bolt. This problem increases as a function of increased bore hole diameter since the volume of the annulus between the roof bolt and bore hole wall increases accordingly because, I believe, of greasing the interface between the mechanical anchor and bore hole wall.
A bail-type mechanical anchor, such as disclosed in U.S. Pat. Nos. 2,952,129; 4,483,645; 4,100,748; and 4,861,148 is typically used in the soft type strata are used. However, when the bail-type anchor is used in conjunction with a quick setting resin a phenomenon called lockup often occurs. Specifically, lockup occurs when the mine roof bolt is placed in a mine roof bore and rotated, generally by a bolting machine, and at a certain point the tension does not increase for a given increase of torque applied to the bolt. Further, after the bolting machine is removed the bolt tends to spring back or rotate in an opposite direction. I believe this is caused by the resin greasing the interface between the mechanical anchor and the mine roof bore wall. This allows the leaves or radially outwardly expandable sections of the shell to slide down the bore hole with the threaded plug or wedge as the bolt is rotated until the plug or wedge reach a non-threaded portion of the roof bolt or a stopping mechanism such as a jam nut. Thus, the result is a less than satisfactory arrangement since the mine roof may not be properly supported by the roof bolts because of the lockup condition.
It is an object of the present invention to provide a roof bolt arrangement that has a mechanical anchor, yet does not exhibit lockup when used in conjunction with a quick-setting resin.
It is a further object of the present invention to provide a roof bolt arrangement requiring less resin than is presently being used in resined mine roof anchor assemblies.
It is still a further object of the present invention to provide an improved roof bolt that can be used with or without resin.